282 LIFE BY THE SEASHORE. 



crossed by fine longitudinal striae. The colour is yellowish 

 white, variegated with reddish brown. There are three 

 cardinal teeth in each valve. A prettier species with more 

 distinctly marked ribs, and streaks and patches of bright 

 colour, often occurs on the shore in the dead condition. 

 This is T. virgineus, which inhabits somewhat deeper water 

 than the common species. The latter lives well in confine- 

 ment, and affords an admirable object for the study of the 

 siphons. When alarmed, the animal suddenly retracts these, 

 producing a very forcible jet of water as it does so ; when it 

 is lying undisturbed the course of the breathing currents 

 can be clearly seen, especially in water containing suspended 

 particles. 



The next sub-order includes the cockles, which have a 

 greatly elongated foot, used in taking "leaps," and also in 

 burrowing in the sand. There are a considerable number of 

 cockles, but as the differences between the species are not 

 very well marked, it may be sufficient if we describe the 

 common, or edible cockle (Cardium edule). This species, as 

 is well known, occurs in beds in sandy and muddy ground, 

 living on, or only slightly below, the surface. It is valued 

 both as food and bait, and is collected by the fisher folk in 

 large quantities, short rakes being used for the purpose. The 

 shell is equivalve, somewhat triangular, and strongly convex. 

 The characteristic cockle appearance is produced by the sculp- 

 ture, which consists of twenty-four to twenty-eight flattened 

 ribs separated by narrow furrows. These ribs project at the 

 margin, as in all cockles, so that the valves lock closely 

 together; in the living animal the mantle is fringed with 

 delicate processes corresponding to these ribs. In an empty 

 shell the internal characters can be made out, the fluted 

 margin, the muscle scars, the strong central (cardinal) tooth 

 in each valve, shaped like a reversed V, and the small 

 laterals at each side of this. The different cockles are 

 distinguished chiefly by the sculpture of the shells, and the 

 number and shape of the ribs; generally speaking all are 

 readily recognised as cockles. 



We come next to two genera whose members show some 

 marked resemblances, combined with distinct differences. In 

 both the large siphons cannot be completely retracted, so that 

 the shell "gapes" permanently, and cannot be closed. 



